feat(src/styles): add dark theme styling and widget CSS

Add CSS for about, battery, calendar, and system info widgets, including layout adjustments, color theming, and interactive component styling.

src/styles/constant.css

@@ -0,0 +1,31 @@
+@define-color bg alpha(#fff, 0.07);
+@define-color hover alpha(#fff, 0.1);
+@define-color border var(--outline);
+@define-color active alpha(#fff, 0.9);
+@define-color urgent alpha(var(--on-error), 0.7);
+
+/* New common colors */
+@define-color accent #007aff;
+@define-color accent-hover #0056cc;
+@define-color bg-lighter alpha(#fff, 0.12);
+@define-color bg-darker alpha(#000, 0.08);
+@define-color text-main #ffffff;
+@define-color text-secondary alpha(#fff, 0.6);
+@define-color text-dim #999;
+@define-color shadow-color alpha(#000, 0.3);
+
+/* Border and Shadow specific */
+@define-color border-subtle alpha(#111, 0.3);
+@define-color border-strong alpha(#111, 0.4);
+@define-color inner-shadow alpha(#000, 0.3);
+
+/* Constants */
+/* :vars replaced by defines */
+    @define radius-xs 4px;
+    @define radius-sm 8px;
+    @define radius-md 12px;
+    @define radius-lg 16px;
+    @define radius-xl 20px;
+    @define radius-full 999px;
+    @define transition-fast 0.15s;
+    @define transition-slow 0.3s;